Identifying the mechanical properties of skin and other biological tissues is important for diagnosing healthy from damaged tissue, developing tissue vascularization therapies, and creating injury repair techniques. In addition, the ability to assess the mechanical properties of an individual's skin is essential to cosmetologists and dermatologists in their daily work. Today, the mechanical properties of skin are often assessed qualitatively using touch. This, however, presents a problem in twins of passing information between different individuals or comparing measurements from different clinical studies for the diagnosis of skin conditions.
Studies have explored both the linear and nonlinear properties of biological materials. Testing methods used include suction (S. Diridollou, et al., “An in vivo method for measuring the mechanical properties of the skin using ultrasound,” Ultrasound in Medicine and Biology, vol. 24, no. 2, pp. 215-224, 1998; F. M. Hendricks, et al., “A numerical-experimental method to characterize the non-linear mechanical behavior of human skin,” Skin Research and Technology, vol. 9, pp. 274-283, 2003), torsion (C. Excoffier, et al., “Age-related mechanical properties of human skin: An in vivo study,” Journal of Investigative Dermatology, vol. 93, pp. 353-357, 1989), extension (F. Khatyr, et al., “Model of the viscoelastic behavior of skin in vivo and study of anisotropy,” Skin Research and Technology, vol. 10, pp. 96-103, 2004; C. Daly, et al., “Age related changes in the mechanical properties of human skin.” The Journal of Investigative Dermatology, vol. 73, pp. 84-87, 1979), ballistometry (A. Tosti, et al., “A ballistometer for the study of the plasto-elastic properties of skin,” The Journal of Investigative Dermatology, vol. 69, pp. 315-317, 1977), and wave propagation (R. O. Potts, et al., “Changes with age in the moisture content of human skin,” The Journal of Investigative Dermatology, vol. 82, pp. 97-100, 1984).
Commercial devices, such as the CUTOMETER® MPA580, DERMALFLEX, and DIA-STRON brand dermal torque meter, exist for some of these methods. Generally, these devices only provide information about limited aspects of skin behavior which may not be enough to properly diagnose disease. Many of these devices also focus on only linear properties such as skin elasticity.
In another method known as indentometry, (F. J. Carter, et al., “Measurements and modeling of human and porcine organs,” Medical Image Analysis, vol. 5, pp. 231-236, 2001; M. P. Ottensmeyer, et al., “In vivo data acquisition instrument for solid organ mechanical property measurement,” Lecture Notes in Computer Science, vol. 2208, pp. 975-982, 2001; G. Boyer, et al., “Dynamic indentation of human skin in vivo: Aging effects,” Skin Research and Technology, vol. 15, pp. 55-67, 2009) a probe tip is pushed orthogonally into the skin to discover tissue properties. If large enough forces are used, this method is capable of measuring the mechanical properties of not only the epithelial layer, but also the properties of the underlying connective tissue.
The interaction between different tissue layers (C. Daly, et al., “Age related changes in the mechanical properties of human skin.” The Journal of Investigative Dermatology, vol. 73, pp. 84-87, 1979; H. Oka, et al., “Mechanical impedance of layered tissue,” Medical Progress through Technology, supplement to vol. 21, pp. 1-4, 1997) is important in applications like needle-free injection (B. D. Hemond, et al., “A Lorentz-force actuated autoloading needle free injector,” in 28th Annual International Conference of the IEEE EMBS, pp. 679-682, 2006), where the dynamic response of skin to a perturbation is important in determining the required injection depth.
Linear stochastic system identification techniques have been used to describe a variety of biological systems (M. P. Ottensmeyer, et al., 2001; G. Boyer, et al., 2009; M. Garcia-Webb, et al., “A modular instrument for exploring the mechanics of cardiac myocytes,” American J. of Physiology: Heart and Circulatory Physiology, vol. 293, pp. H866-H874, 2007). However, many systems cannot be fully described by linear dynamic models. Investigators have also used nonlinear relationships to describe the stress strain relationship in skin (F. M. Hendricks, et al., 2003). However, most of this work has been done at low frequencies and therefore does not describe the dynamic properties of skin.
The frictional properties of skin have been studied by several research groups (A. F. El-Shimi, “In vivo skin friction measurements.” Journal of the Society of Cosmetic Chemists, 28:37-51, 1977; W. A. Gerrard, “Friction and other measurements of the skin surface.” Bioengineering and the Skin, 3:123-139, 1987; R. J. Hills, A. Unsworth, and F. A. Ive, “A comparative study of the frictional properties of emollient bath additives using procine skin” British Journal of Dermatology, 130:37-41, 1994; S. Nacht, J. A. Close, D. Yeung, and E. H. Gans, “Skin friction coefficient: changes induced by skin hydration and emollient application and correlation with perceived skin feel.” Journal of the Society of Cosmetic Chemists, 32:55-65, 1981; M. Zhang and A. F. T. Mak, “In vivo friction properties of human skin.” Prosthetics and Orthotics International, 23:135-141, 1999) using custom and commercial instruments, such as the MEASUREMENT TECHNOLOGIES Skin Friction Meter (Aca-Derm Inc., California). On the other hand, the surface mechanics of skin includes several properties including skin texture, suppleness, and friction (Zang and Mak, 1999). When a probe is placed on the skin, the skin can deform contributing to changes in the measured damping (from skin energy absorption, skin friction and other factors) as well as the spring constant (from skin suppleness, skin stiffness and other factors).
Another problem with existing methods is that the dynamics of the testing device are often not characterized and are assumed to apply perfect forces to the tissue. For example, actuators are assumed to have perfect output impedance such that the dynamics of the system being tested do not affect the dynamics of the actuator. In addition, many existing methods and devices are limited to one test geometry and one perturbation scheme. Once a different geometry or testing direction is used, the measured results are not easily comparable.
Trends in consumer skin care have shown the use of specific molecules and proteins, such as tensin, which are well known to cause collagen growth or increase skin suppleness in hydration and anti-aging products. Although standard testing devices for skin have been proposed, industry specialists have expressed dissatisfaction with existing devices.